1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to a system for cleaning lithographic printing members following digital imaging.
2. Description of the Related Art
In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording is medium in the imagewise pattern with substantial fidelity. Dry printing systems utilize printing members whose ink-repellent portions are sufficiently phobic to ink as to permit its direct application. Ink applied uniformly to the printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or "fountain") solution to the plate prior to inking. The ink-adhesive fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
To circumvent the cumbersome photographic development, plate-mounting and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers. For example, U.S. Pat. Nos. 5,351,617 and 5,385,092 disclose an ablative recording system that uses low-power laser discharges to remove, in an imagewise pattern, one or more layers of a lithographic printing blank, thereby creating a ready-to-ink printing member without the need for photographic development. In accordance with those systems, laser output is guided from the diode to the printing surface and focused onto that surface (or, desirably, onto the layer most susceptible to laser ablation, which will generally lie beneath the surface layer).
Many kinds of plates imageable by laser or other recording instrument, and particularly those involving ablation mechanisms, generate debris. For example, some of the plates described in U.S. Pat. Nos. 5,339,737 and 5,379,698 include a topmost silicone layer, an underlying layer ablatable by laser discharge, and a strong, stable substrate beneath the ablation layer. Exposure of the plate to a laser pulse destroys the ablation layer, weakening the overlying silicone layer and de-anchoring it. The silicone layer is not, however, removed by imaging. Accordingly, after the plate has been fully scanned by the laser, the disrupted silicone must be removed.
Various approaches have been suggested for removing plate debris produced in the course of platemaking, and specifically in connection with imaging processing involving ablation. One such cleaning system is disclosed in U.S. Pat. No. 5,148,746. Basically, that system comprises a rotating brush affixed to the writing head that can be moved into contact with the surface of the lithographic plate undergoing imaging. While that prior plate-cleaning apparatus operates satisfactorily in many respects, it is relatively slow because the brush cleans only a relatively small area of the plate at any given time. In other words, the brush head must be gradually moved along the entire length of the plate cylinder as it rotates in order to clean the entire surface of the plate. Other cleaning systems for digitally imaged lithographic printing plates are disclosed in U.S. Pat. No. 5,568,768 and copending application Ser. No. 08/756,267.
Particularly in on-press implementations, the cleaning system should be capable of unobtrusive integration within the imaging environment, in terms of both function and structure. That is, operation of the cleaning system should not interfere with the imaging process, and the components of the cleaning system may desirably be mechanically separate from the imaging components. This arrangement would permit the cleaning system to be separately serviced, and also help to avoid unwanted mechanical interactions between imaging and cleaning elements. Indeed, in an on-press configuration, it is generally useful to isolate the cleaning system from ink-transfer components as well.